IMPACTS OF THE 2000-2022 MULTI-YEAR DROUGHT ON HYDROLOGY IN THE COLORADO RIVER BASIN USING A HIGH-RESOLUTION, GRID-BASED WATER BALANCE MODEL

A multi-decadal drought in the Colorado River Basin (CRB) caused by decreasing precipitation and increasing air temperature is impacting water availability on a regional scale. To analyze the effects of the current drought relative to past conditions, the Basin Characterization Model (BCM) was used to simulate the monthly water balance using a 270-meter grid (about 9 million cells in the CRB). The BCM uses an energy-balance approach to simulate potential evapotranspiration (PET). Actual evapotranspiration (ET), soil moisture, snow accumulation, snow melt, recharge, runoff, and climatic water deficit (CWD, defined as PET minus ET) are simulated using climate inputs, simulated PET, calculated soil storage capacities, estimated bedrock and alluvium permeability, and monthly vegetation parameters. Climate inputs for October 1896 to May 2022 were developed using gridded monthly (~4-km) PRISM precipitation and maximum and minimum monthly air temperature data, downscaled to the CRB. Parameters defining physical basin properties were estimated using available geospatial data. Parameters affecting snow accumulation and melt, ET, and subbasin discharge were calibrated using snow water equivalent (SWE) data, published monthly ET maps, and streamflow data. Model results were used to identify differences in the current drought’s hydrologic impacts on the landscape relative to historical dry periods and to quantify spatial variations in the severity of drought impacts throughout the CRB. Monthly drought indices and cumulative departures from long-term means were calculated for SWE, CWD, runoff, and recharge. Preliminary results showed the 20-year mean available water (recharge + runoff) at the end of water year (WY) 2021 declined by 16% for the upper CRB and by 21% for the lower CRB compared to mean results for WYs 1981-2010. Consistent trends of increasing CWD and decreasing SWE starting from about WY 1991 for the Lower CRB and WY 1999 for the upper CRB were identified. Updated results for WY 2022 (through May) indicate some mitigation of drought impacts due to increased precipitation relative to WY 2021; however, 10- and 20-year mean recharge and runoff rates for subbasins throughout the CRB showed continued multi-year decreases in available water relative to conditions prior to about WY 1993.